The choice frequencies at different light intensities for two horses (A) and six human subjects (B) as a comparison (binominal tests *P<0.05).

<p>The data presented for Chap (black bars) and Rosett (grey bars) start from the second day they performed at least 70% correct choices and the number of choices are written above each bar. Chap discriminated colours at 0.02 cd/m² but chose randomly at 0.007 cd/m². Six human subjects (B) were presented with 20 combinations per intensity. Initials of the human subjects are listed from left to right as they appear in the graph. All but RF chose the positive stimulus significantly more often at 0.02 cd/m². At 0.007 cd/m² all but AB failed to discriminate the colours. AB did not fail until the light intensity was lowered to 0.003 cd/m².